How a ruler will be chosen in the Khilafah state ?

We can VOTE for our ruler as long as he rules by what Allah has revealed. Voting is not the same as democracy. Voting means to elect an individual to do an action on your behalf – i.e. to represent you or, in Arabic, to be your wakeel. Representation or wakala has an established set of rules in Islamic jurisprudence as we said before the key question is: what will the person represent you in doing.
In the Second Bai’ah (pledge) of Al-Aqabah, the Messenger (saw) said to leaders of the tribes of al-Aws and al- Khazraj (mentioned in Seerah of Ibn Hisham): “Select for me from amongst you twelve chiefs, who will be responsible for their people, including themselves…” This means he asked them to select and elect their representatives.

Also, in the example of the selection of the Khaleefah at the time of the passing of Ameer al Mu’mineen Sayyidina Omar ibn Khattab (RA) a group of 6 were chosen by him, on his death bed, to select/elect from amongst them the new Khaleefah – and he instructed to kill anyone who opposed the matter after 3 days and 2 nights – an instruction agreed upon by the Sahabah, which established an ijma of obligation of this matter of electing a Khilafah within this time. In the end, the people of Medina were consulted about the two nominees from this group and Sayyidina Uthman ibn Affan was elected.

Election process for the Khalifah will be managed by the body known as the Majlis al Ummah – or the Ummah’s council. This body is elected from the people, by an election process and voting so the people chose their real representatives. Then these representatives would be responsible for reducing a list of nominees to 6 final candidates – checked by the 'Mahkamat al Mathalim' – the Judge who would establish that they fulfilled the criteria for ruling. The Majlis would then elect the Khalifah from this shortlist and give the bay’ah. This is the practical manifestation of the principle that the authority is with the Ummah.

Thus, in the Islamic system, Sovereignty is for Allah and His Shariah alone – and authority is with the Ummah.